this is my 4rth wbody coupe.they all get low 30'* even with 300k. i had a 88 cutlass put over 200k never touched the engine or trans. i had a 90 gp put over 300k never touched the engine but the trans once. then i had a 03 monte 3.4 that got 34 new and i put a k&n drop in it e3 plugs msd wires and strait piped the silencer i got 38 a few times that way average 36. put 170k on it and found this very well taken care of 07 3500flex. i switched jobs and have only a rural 2 lane commute now but doing that with only a k&n i get 32 no a/c 30 with a/c on.
in between there i had a neon es i bought new i thought the 4 cyl would get better mileage but it didnt, cause they are geared for city and the mileage actually falls off quite fast over 55, where the gm 6'* get there best between 60-65
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock